Weight painting problem with human head
Not really presented as an answer, but I wanted to add these images to my comment above:

Looks like maybe that part pf the mesh does not completely link to the top bone? Facial rigs often have a primary bone with subordinate children bones that use copy rotation and copy location modifiers to allow motion more complicated than can be achieved by one bone alone. This may not be the answer, but if you post a screen shot of your weight painting, we could tell...





These images are not my work and are used for educational and explanatory purposes only.